Generally, the seaming of a can body in a three piece can consisting of two can ends and a single can body is carried out by soldering, adhesion with a nylon adhesive and electric welding.
Recently, electric welding has been widely used for the seaming of the tinplate can body in the field of food cans, aerosol cans and miscellaneous cans, instead of soldering with a solder of regulated lead content. In the seaming of the tinplate can body, it is desirable to decrease the tin coating weight in tinplate, because tin used for the production of tinplate is very expensive. However, the weldability of tinplate gradually becomes poor with a decrease of the tin coating weight.
From the background described above, the development of a welded can material, which is cheaper than conventional electrotinplate, is easily welded without removing the plated layer at high speed and is excellent in lacquer adhesion and corrosion resistance after lacquering, has been required in the field of food cans.
Within the last few years, various surface treated steel sheets have been proposed as welded can materials having the characteristics described above. For instance, low tin plated steel sheet (LTS) with below about 1000 mg/m.sup.2 of tin which is reflowed or unreflowed after tinplating has been proposed. However, this LTS has a narrower current range for sound welding than that for tinplate. The reason is considered to be that the amount of free metallic tin in this LTS is smaller than that in tinplate and also further decreases because of the change of plated free metallic tin to iron-tin alloy by heating for lacquer curing or reflowing after tinplating. For the improvement in the weldability of this LTS, the following three methods have been proposed. The first method is one in which a steel sheet is plated with a small amount of nickel before tinplating. In this method, a decrease in the amount of plated free metallic tin, that is, the change of plated metallic tin to iron-tin alloy by heating for lacquer curing, is suppressed because a dense nickel-tin alloy layer formed during aging at room temperature or a dense iron-tin alloy containing nickel formed by reflowing after tinplating, acts as a barrier for the diffusion of iron to plated tin. The second method is one in which nickel is plated on a steel sheet before annealing and then all or a part of the plated nickel is diffused on the surface of the steel sheet by heating for the annealing of the steel sheet, after which a small amount of tin is plated on the steel sheet covered with a nickel diffusion layer. The third method is one in which tin is plated on a steel sheet before annealing, instead of nickel in the second method.
In the second and third methods, a nickel diffusion layer or an iron-tin alloy layer formed on the steel sheet by heating for the annealing of the steel sheet acts as a barrier for the change of the plated metallic tin to iron-tin alloy by heating for the lacquer curing or reflowing after tinplating.
Although the weldability and the corrosion resistance after lacquering of the LTS by these methods described above are improved, the excellent lacquer adhesion required for a can material is not obtained. The reason is considered to be that the surface of the LTS is oxidized during aging in an ordinary atmosphere because the surface of the LTS is not sufficiently covered with the film formed by an electric chromic acid treatment. If the surface of the LTS is sufficiently covered with this film, the weldability becomes poor, although the lacquer adhesion of the LTS may be improved.
Accordingly, it is the first objective of the present invention to provide a surface treated steel sheet having excellent weldability, excellent lacquer adhesion and excellent corrosion resistance after lacquering for a welded can material.
It is the second objective of the present invention to provide a method for the continuous production of a surface treated steel sheet having excellent characteristics as described above.